New York City’s air was “very unhealthy,” Adams said.

Mayor Eric Adams said in a news release late Tuesday that by 10 p.m. air quality in parts of New York City had become “very unhealthy,” rising to 218 on the Environmental Protection Agency’s Air Quality Index.

New York’s air quality rating briefly ranked as the worst of any city in the world on Tuesday, according to the IQAir World Air Quality Index. By comparison, the city’s air quality has generally been below 50 on the index in recent years, in the “good category,” and even improved during the pandemic-driven lockdown in 2020, according to IQAir.

“While conditions are anticipated to temporarily improve later tonight through tomorrow morning, they are expected to deteriorate further tomorrow afternoon and evening,” Mr. Adams warned. He noted that the state’s Department of Environmental Conservation had issued a citywide air-quality health advisory.

The mayor indicated that the city’s schools would be open on Wednesday and he urged all students to attend, but he added that schools would not be holding outdoor activities. About 10 Public Schools Athletic League soccer, baseball and softball games had been scheduled for Wednesday.

“We recommend all New Yorkers limit outdoor activity to the greatest extent possible,” Mr. Adams said. “Those with pre-existing respiratory problems, like heart or breathing problems, as well as children and older adults, may be especially sensitive and should stay indoors at this time.”

The mayor said he would brief the public on his administration’s plans Wednesday morning.